1. Field of the Invention
The present invention relates generally to medical methods and devices, and more particularly to a method and apparatus for forming vascular prostheses from host tissue sources.
Coronary and peripheral atherosclerosis are characterized by partial or total occlusion of the arteries resulting from the accumulation of lipids, smooth muscle cells, connective tissue, and glycosaminoglycans on the arterial wall. Atherosclerosis of the coronary arteries is a particular problem and can cause angina and myocardial infarction (heart attack). Although many coronary lesions can be treated with percutaneous techniques, such as angioplasty and atherectomy, more tortuous and severely diseased arteries frequently require surgical intervention and bypass, commonly referred to as coronary artery bypass graft (CABG) surgery.
CABG surgery relies on the surgical attachment of a vascular graft to bypass the arterial occlusion in order to restore blood flow to the coronary vasculature. The nature of the vascular graft can have a significant impact on the ultimate success of the procedure. A preferred vascular graft is formed from autologous internal mammary artery (IMA), where the resulting grafts have a patency rate approaching 95% ten years following the procedure. The use of IMA grafts, however, is limited by their length, and the need to harvest the artery from the patient can result in post-surgical complications. The autologous saphenous vein is a second common source for vascular grafts. While generally available in the necessary lengths, the saphenous vein is not ideally suited for replacement as an arterial vessel, and patency rates at ten years are often below 50%. Moreover, removal of the saphenous vein from the leg can also cause post-surgical complications.
Because of the limitations on autologous vascular sources, a variety of synthetic and non-autologous biological prostheses have been proposed. Common synthetic prostheses are formed from Dacron.RTM. and PTFE, and can perform well when employed in larger diameters, i.e., above 6 mm. Smaller synthetic prostheses, however, occlude at a relatively high rate. Non-autologous biological conduits which have been utilized as vascular prostheses include human umbilical vein grafts and bovine internal mammary arteries. Synthetic grafts have also been seeded with human and other mammalian cells or proteins, e.g., collagens, in an effort to improve their long-term patency rate. Presently, however, none of these approaches has demonstrated long-term patency, particularly in smaller diameter grafts.
Of particular interest to the present invention, preparation of vascular prostheses from autologous pericardium has been proposed. Pericardial tissue is harvested from the patient and formed into a tubular graft by suturing along a longitudinal line. While promising, the use of sutures can result in an irregular seam which, in turn, can cause turbulent blood flow and result in clot formation. Moreover, such grafts are unsupported and subject to kinking and collapse. The grafts further lack an inherently round geometry and are subject to dimensional changes, e.g., elongation and aneurysmal formation. Because of the dimensional uncertainty, it is difficult to match such grafts to the precise dimensional requirements of the particular application, e.g, caliber and length. The suturing of vascular prostheses from pericardium is labor intensive and time consuming, and the resulting structures are subject to rupture and other structural failure. Thus, the outcome of using sutured pericardial tissue grafts is uncertain at best.
A significant improvement over such prior autologous pericardial graft structures is disclosed in copending application Ser. No. 08/580,582, filed on Dec. 29, 1995, and assigned to the assignee of the present application, the full disclosure of which is incorporated herein by reference. In that copending application, a prosthetic graft is formed by wrapping a sheet of tissue over a first helical frame which is optionally supported on a mandrel. A second helical frame is then placed over the tissue-wrapped mandrel to complete the graft structure. No suturing is necessary, and the resulting structure is dimensionally stable, available in a variety of lengths, and biologically compatible.
Despite such advantages, the graft structures described in the copending application can be cumbersome to fabricate, particularly when the fabrication is carried out in the operating room after harvesting of the tissue and before performance of a CABG or other grafting procedure. For example, placement of the outer helical frame over the tissue-wrapped inner frame can be difficult, particularly when trying to properly align adjacent turns of the outer helical frame between corresponding turns of the inner helical frame. Removal of the completed graft structure from the assembly mandrel can be difficult. Both the helical frames and the tissue are at risk of damage, and it is difficult to achieve repeatable, consistent graft structures.
For these reasons, it would be desirable to provide improved methods and apparatus for forming tubular prostheses from patient tissue over tubular support frames. It would be particularly desirable to provide methods and apparatus which permit the rapid placement of such helical outer frame components over an assembly mandrel with a high degree of accuracy and repeatability, and which further permit and facilitate removal of the fully formed prosthesis from the assembly mandrel without damage to either the tissue or the helical support structures. Such methods and systems should be suitable for preparing tubular prostheses having a wide range of diameters and lengths, and should be relatively easy to use while minimizing any chance of improper use. The present invention will address at least some of the objectives set forth above.
2. Description of the Background Art
U.S. Pat. No. 4,502,159, describes a vascular prosthesis made by suturing glutaraldehyde-treated pericardial tissue along a longitudinal seam. SU 1217362 (Abstract) describes reinforcing arteries by securing pericardial tissue over the artery. U.S. Pat. No. 3,562,820, describes forming tissue-containing prostheses over removable mandrels. The use of glutaraldehyde and other agents for treating tissue and prosthetic devices to reduce antigenicity is described in U.S. Pat. Nos. 3,988,782; 4,801,299; 5,215,541, and Brazilian applications 89/03621 and 90/03762. U.S. Pat. No. 4,539,716, describes the fabrication of an artificial blood vessel from collagen and other natural materials. U.S. Pat. Nos. 3,894,530 and 3,974,526, describe the formation of vascular prostheses from the arteries or veins present in the umbilical cord. U.S. Pat. No. 5,372,821, describes the use of tissue for forming artificial ligament grafts for use in orthopedic procedures. U.S. Pat. No. 3,408,659, describes the preparation of vascular artificial prostheses from other body lumens. French application FR 2,714,816, (Abstract) discloses a helically supported vascular prosthesis. A number of medical literature publications describe the use of vascular prostheses formed form tissue. See, for example, Rendina et al. (1995) J. Thorac. Cardiovasc. Surg. 110:867-868; Hvass et al. (1987) La Presse Medicale 16:441-443; Allen and Cole (1977) J. Ped. Surg. 12:287-294; and Sako (1951) Surgery 30:148-160. Other patents and published applications relating to synthetic vascular grafts include U.S. Pat. Nos. 4,728,328; 4,731,073; 4,798,606; 4,820,298; 4,822,361; and 4,842,575; and PCT publications WO 94/22505 and WO 95/25547. Patents and published applications relating to kits for preparing replacement heart valves from pericardial and other autologous tissue sources are described in U.S. Pat. Nos. 5,163,955; 5,297,564; 5,326,370; 5,326,371; 5,423,887; and 5,425,741.